This application is related to Japanese application No. 2000-367456 filed on Dec. 1, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a magnetooptical recording medium (hereinafter referred to as xe2x80x9cmediumxe2x80x9d) and a reproduction method thereof. The magnetooptical recording medium of the present invention can be suitably applicable to Partial Response Maximum Likelihood (PRML) technology.
2. Description of Related Art
The magnetooptical recording medium is known as xe2x80x9ca high density recording mediumxe2x80x9d, but further densification has been requested together with an increase of information output. A method of shortening the mark length is shown for carrying out the further densification, and therefore a magnetic super-resolution technology has been adopted. Various proposals are made as said magnetic super-resolution technology, but of them the center aperture-type magnetic super-resolution (CAD) medium (e.g. Refer to Japanese Unexamined Patent Publication No. 9 (1997)-320134) and Double Mask-Rear Aperture-type magnetic super-resolution (D-RAD) medium (e.g. Japanese Unexamined Patent Publication No. 9 (1997)-147436) have been noted. Hereinafter both media and a reproduction method of it will be explained in reference to FIGS. 1(a) and (b). Further, the medium construction is shown at the upper part of FIGS. 1(a) to (c), and the regenerated wave shape is shown at the lower part.
As shown at first by FIG. 1(a), in the former CAD method is normally used the magnetooptical recording medium composed of the recording layer 1, non-magnetic layer 2, intermediate layer 3 and reproduction layer 4, in which the reproduction layer 4 and intermediate layer 3 show an in-plane direction as magnetizing direction, and the recording layer 1 has a perpendicular magnetizing direction. When light is irradiated for reproducing the information recorded in this medium, the magnetizing direction of the recording layer is transferred to the reproduction layer in the center of the light spot 5 (high-temperature part). Around the surroundings within the light spot (low-temperature part), the magnetizing direction of the reproduction layer turns to a slant direction depending upon the influence of magnetization of the recording layer to produce a magnetic mask. The reproduction signal in reproducing the magnetic super-resolution takes almost a sine wave. Further, the arrow mark A in the figure means a moving direction of the light spot.
As shown in FIG. 1(b), in the latter D-RAD method is normally used the magnetooptical recording medium composed of the recording layer 1, intermediate layer 3 and reproduction layer 4, but the magnetizing direction of each layer is perpendicular. In irradiating light for reproducing the information recorded on this medium, the magnetizing direction of the recording layer is transferred to the reproduction layer in the center of the light spot 5 (medium-temperature part). Around the surrounding within the light spot 5, the magnetic mask is formed in the high-temperature side of the left-hand on the figure and in the low-temperature side of the fight-hand on the figure to regenerate the magnetic super-resolution, depending upon the influence of the reproducing magnetic field. Further, the blank part of the intermediate layer 3 is heated at temperature more than the Curie temperature, while showing the state of non-magnetization. This method has no part in which the magnetizing direction faces a slant direction in the reproduction layer as in CAD method. The reproduction signal during the reproduction takes so steep a change on the wave shape in comparison with that of CAD method as to produce a distorted wave shape far from the sine wave. This signal changes so steep, because the magnetizing direction to be a mask in the low-temperature side turns downward and the magnetizing direction in the part to be transferred turns upward.
As shown from the figures, D-RAD method takes a bigger amplitude than that of CAD method. In other words, this means a high resolution capacity. Therefore, D-RAD method has a better C/N than that of CAD method.
However, CAD method may improve C/N by about 2 dB by using a signal treating process customarily called PRML technology. This technology is based on the fact that the reproduction wave shape has a sine wave, but it is difficult to apply PRML technology in the case of D-RAD method.
As the result of keen investigations, the inventor has found that, in view of the advantages and problems of said D-RAD method and CAD method, the reproduction wave shape in the medium used in D-RAD method is almost close to the sine wave, thereby the medium is able to apply PRML technology.
Accordingly, the present invention is to provide a magnetooptical recording medium comprising at least a reproduction layer, an intermediate layer, a connection layer and a recording layer, wherein the reproduction layer and intermediate layer have a slant magnetic direction in a non-magnetic field, and the connection layer is composed of a layer non-magnetic at room temperature by itself which is induced to exhibit magnetism by contact with a magnetic layer.
Further, the present invention is to provide a magnetooptical recording medium comprising at least a reproduction layer, an intermediate layer, a connection layer and a recording layer, wherein the reproduction layer and intermediate layer have a shift amount of 200xcx9c50 Oe in absolute value and the connection layer is composed of a layer non-magnetic at room temperature by itself which is induced to exhibit magnetism by contact with a magnetic layer.
Further, since the regenerated wave shape is near a sine wave, PRML technology may be applied so that there can be realized C/N bigger than that of the medium to be used in D-RAD method.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.